power amp influence on sound?

[J M Fahey]

I make PCBs at home, but I'm making them every week, and sell the amplifiers so I had to cut costs down to remain competitive.
That justified buying the necessary stuff and learning to use it.
So there are different options but "best" depends on how many you will make and for how long .

1) If you want to test your concept, maybe sell 1 or 2 to test the waters, go for a DIY oriented commercial fabricator, such as Express PCB:
http://www.expresspcb.com/ExpressPCBHtm/SpecsStandard.htm
Their service is incredibly useful for Robotics guys, Microprocessor users, etc. where a small but very complex PCB is needed.
Problem for us Music Instrument Amp makers is that we use large simpler PCBs and they charge by the square inch, so costs go through the roof quickly

2) Next best for small volume is using precoated photosensitive PCBs.
http://www.mouser.com/Tools-Supplies/PCBs-Circuit-Boards/_/N-8vofl?P=1yzvq68
Incredible Pro quality, sharp thin lines, you can leave the photosensitive coat on , which is solderable and protects the copper from oxidation, the works.
The raw board 300x450mm (about 12" x 18") which you can cut to size is more in line with what we need.
You'll need to design and draw your PCB artwork, print it on transparency or "vellum" (the semi transparent drawing paper used by Architects and Engineers) and photo expose that on the PCB sensitive surface.
Look at this tutorial:
http://sfprime.net/pcb-etching/index.htm
It's too corner cutting, he prints on regular paper and greases it with cooking oil instead of using the proper transparent papers, but it will give you an idea.

3) For small scale production at hobby level, thermal toner transfer is fine and inexpensive, but you must  look very carefully at the printed PCB and correct with Sharpie , it's very easy to have small cracks or even parts of the tracks missing, because at some spot the toner didn't stick very well to the copper.
There's TONS of how-to guides, search for them, and practice practice practice
http://www.instructables.com/id/Cheap-and-Easy-Toner-Transfer-for-PCB-Making/
http://www.pcbfx.com/main_site/pages/products/transfer_paper.html

4) If you will be making them regularly, or need, say, 20 or more PCBs, learn silkscreening (whay I use) but you need to be really committed to it, spend some $$$ and learn to use it , practice makes perfect, so it really depends on how many you want to make.

If you have a friend who silkscreens TShirts and such, he already has most of the stuff plus the required skills, so if you buy the stuff he does not have (PCB printing "ink" is different , etc.) he might print them for you.

5) of course you'll have to download a PCB drawing program and learn to use it.
Eagle is popular, but the free version isn't that great for us, it's also geared for Microprocessor guys and such and allows for a too small PCB.
Download and try Express PCB even if they won't make your PCBs, it's relatively simple to learn and you can print your final drawing and use it for toner/photosensitive/silkscreen. Cool.
Only gripe I have with it, is that it's preset for too small pads and thin PCBs, we use much larger because of higher currents and mechanical vibration.
But it's a good learning tool.

6) last but not least: the ***OLD*** technique (I started using it ) of printing your design on regular paper, then you cut it, tape it over the copper side as a guide, and prick through each component hole with a sharp needle, as to mark its position on the copper.
Then you pull the paper and hand draw the lines with waterproof Sharpie or similar.
Crude but cheap and easy.
In this case you needn't even draw and print the proper schematic using a PCB, a 1:1 pencil drawing on any paper is enough.
Did I say **CHEAP**?

[phatt]

Hi add4,
What J M Fahey did not say is how many hundreds/ (make that thousands) of hours it took just to get where He is now. 8|

The best thing you can do is start with small pedal type circuits ,, simple and low voltage is safe for those wishing to indulge in electronics.

Better still purchase a solder less bread board setup and just plug in a few ideas and see what works.

For home hobby types,,By far the most expensive part is the box and the fancy front panels,, if that is the direction you wish to go.

For me I've got no labels on myself and I don't care much for fancy art work as the TONE is what I want not the name or the fame.

I come in at number 6 of Fahey's ideas on how to make PCB stuff. :lmao:
For those starting out it works and does not cost a fortune to make at home.
As long as you don't go silly and try to make incredibly complex PCB's with insanely thin tracks it will work fine.
My first dozen PhAbbTone circuits where made by hand in the manner mentioned above.
I've not had one complaint from those that have purchased my home made handy work.

The biggest mistake you can make is to build yourself into a corner,, meaning building stuff you can't fault find because you do not have the technical ability to understand how it works.
(I am also guilty of that mistake)

As to chassis,, The trick is to find an industrial estate with Sheet metal workshops.
You raid their waste bins,, you can get lucky and find off cuts that are often close to what you need. (Wise to ask first)

I'm lucky because I spent most of my working life in the metal industry so I already know what to look for.

Failing that the net may well contain many ideas of how to work with sheet metal.

Steel is ok at 1.2 mm but over 1.6 it becomes a lot harder to bend at home.
2 mm is tough and is better left to the experts.

For the novice picking gauge of metal in a scrap bin will be tricky but it won't take long to note the subtle difference between 1.2 mm and 1.6 mm.

Aluminum is great stuff but a little harder to find as scrap and you only get one go at a 90 deg bend. 
Phil.

[teemuk]

In general, are power amps transparent and does the color of the amp comes from the preamp?

Depends. Many guitar amps are intently designed to NOT BE transparent. This including the Polytones.

As you can examine from the schematics the power amp includes a switch to toggle or disable the current feedback path. (As you can see, the speaker's negative terminal does not connect directly to ground, instead load current is converted to voltage by a low-ish resistance - two 0.3 -ohm resistors in parallel - and then fed back as another negative feedback input). This will consequently increase the amp's output impedance resulting into decrease of damping factor and variation in frequency response when the amp drives a reactive load such as a typical loudspeaker.

Disable this current feedback path and the amp will have a flat frequency response, like generic HiFi or PA amps that are - for understandable reasons - designed to not color the tone. Enable the (current) feedback path and the amp will begin to have more gain at higher load impedances, which will effectively provide a boost at speaker's resonant frequency and at higher frequencies where coil inductance increases the impedance. This is the kind of colouration introduced by many amps of low damping nature - most prominently tube amps.

So I'd say: with a good chance a modern solid-state guitar power amp IS NOT TRANSPARENT and will provide some forms of tone altering. Most of them use at least this particular scheme (and many today feature diode clippers for distortion or elaborate tube power amp emulation schemes e.g. Peavey's TransTube amps, Vox's Valvereactor amps etc.) The current feedback scheme in all it's simplicity - also found from many Polytone amps - has been featured in many transistor power amps since mid 1970's. Earliest examples actually even date back to earliest history of transistor guitar amps to begin with. IMO, these things have never been something that could be universally considered "transparent" or "non-colouring". Some may be, most aren't.

[Kaz Kylheku]

The power amp has a huge influence on sound, solid state or not.

A solid state voltage amplifier (global negative feedback based on output voltage) sounds nothing like a similar (or the same) amplifier in with mixed mode feedback (global negative feedback based on a mixture of the voltage applied to the speaker and the amount of current flowing through the speaker). The difference in response cannot be obtained with an equalizer. Creating the tone of mixed feedback in the preamp and then using a voltage amplifier falls into the territory of emulation, raising issues of whether it is true to life, whether it sounds right at full volume, etc.

Moreover, power amps can have tone controls which are in the feedback loop.  If there is current feedback, then these controls interact with the speaker, and so they "do things" that a preamp EQ does not. For example, a bass control in the negative feedback loop will increase speaker resonance, which is not the same thing as equalizing for more bass.

Besides types of negative feedback we could probably talk about other possible effects that could happen in power amps, but that's one big one.